Log error instead of warning, when batch population fails

user experience does not change but should help the admin to track
down the problem quicker..

tailbone/views/batch/core.py

@@ -1057,7 +1057,7 @@ class BatchMasterView(MasterView):
             self.handler.do_populate(batch, user, progress=progress)
         except Exception as error:
             session.rollback()
-            log.warning("batch population failed: %s", batch, exc_info=True)
+            log.exception("batch population failed: %s", batch)
             session.close()
             if progress:
                 progress.session.load()